1. Field of the Invention
The present invention relates to the field of lithography. In particular, the present invention provides a device for optimising the uniformity of an image in a lithographic projection system.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics and catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles, for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Any refractive, reflective or catadioptric elements in the radiation or illumination systems may be based on a substrate of glass or another suitable material, and may be provided with either single- or multi-layer coatings as desired. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO98/28665 and WO98/40791.
In lithography apparatuses, a mask (reticle) is irradiated. The mask has a pattern made from regions which either transmit radiation or block radiation; alternatively, so-called phase modulation (PSM) may be employed. The pattern on the mask is thus projected onto a substrate, typically of semiconductor material. The substrate (e.g. a wafer) is covered with a radiation-sensitive layer. Hence, the pattern on the mask is transferred onto the substrate.
A photolithographic apparatus of the construction described above and being usable, inter alia, in the manufacture of integrated circuits (ICs), is known, for example, from U.S. Pat. No. 5,194,893.
Due to the demand for an increasingly large number of electronic components in one IC, increasingly smaller details, also referred to as line widths, must be imaged by means of the projection apparatus in each area of the substrate on which an IC must be formed, which area is also referred to as IC area or xe2x80x9cdiexe2x80x9d. One also wants to enlarge these IC areas so as to increase the number of components per IC. For the projection lens system this means, on the other hand, that the resolving power, hence its numerical aperture, must be raised and, on the other hand, that the image field must be enlarged. It is difficult to unite these two disparate requirements in a conventional lens.
This dilemma can be circumvented by changing from a so-called step-projection apparatus to a step-and-scan apparatus as described in U.S. Pat. No. 5,194,893 for example. In a step-projection apparatus, the complete mask pattern is illuminated and imaged as a whole on a target area on the substrate, for example, a wafer die or IC area. Subsequently, a step is made, i.e. the substrate is moved with respect to the projection lens system and the mask pattern, until a second target area is present opposite the mask pattern and within the image field of the projection lens system, and a second image of the mask pattern is formed in that area. Subsequently, the apparatus steps to a third target area and the mask pattern is imaged again, and so forth, until images of the mask pattern have been formed in all target areas.
In a step-and-scan apparatus, the same stepping movements are performed, but each time only a small part of the mask pattern is imaged on a corresponding sub-area of the target area. By imaging successive parts of the mask pattern on successive sub-areas of the target area in a scanning motion, an image of the complete mask pattern is obtained on a target area.
To this end, the pattern on the mask is illuminated with a projection beam which forms a small, for example, rectangular or arcuate illumination spot at the location of the mask pattern. In order to move the mask and the substrate, the mask is held on a mask table and the substrate is held on a substrate table. The mask table and the substrate table are moved (scanned) in the same direction or in mutually opposite directions along the scan direction, with respect to the projection lens system and the projection beam, the speed of the substrate being M times that of the mask table, where M is the magnification with which the mask pattern is imaged onto the substrate.
A commonly used value for M is {fraction (1/4)} or ⅕. Other values of M, for example 1, are alternatively possible. Said movement of the mask table and the substrate table with respect to the illumination spot is referred to as the scan movement. The illumination spot has its largest dimension in the direction transverse to the scan direction. This dimension may be equal to the width of the mask pattern, so that this pattern is imaged in one scan movement. However, it is alternatively possible that said dimension is half the mask pattern width or even smaller. In that case, the complete mask pattern can be imaged by performing two, or a larger number of, opposed scan movements.
It should then be ensured that the mask and the substrate have the correct mutual position and speed at any moment, which can be realised by means of a very accurate synchronisation of the movements of the mask and substrate tables, i.e. the speed Vsub of the substrate should always be equal to M times the speed vma of the mask.
To this end the apparatus may, for example, comprise a first and a second interferometer system for continuously measuring, during each imaging operation, the mutual position of the mask and the substrate.
Lithographic projection apparatuses may employ a projection beam of electromagnetic radiation, such as UV radiation, e.g. with a wavelength of 365 nm, 248 nm, 193 nm or 157 nm, or extreme UV, with a wavelength of the order of 15 nm. Alternatively such apparatus may employ a projection beam of charged-particle radiation, such as electron radiation or ion radiation, in which case an associated field-lens projection system is used.
More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
There is a need to produce smaller and smaller semiconductor devices, and thus a corresponding need to improve the so-called critical dimension (CD) uniformity. Thus, these lithography apparatuses are being pushed to their resolution limits. Therefore, there is a need to minimise factors which can affect the resolution of the apparatus.
It is critical to produce a uniform light distribution at substrate level to achieve a high CD uniformity at high resolutions. It is an object of the present invention to produce a uniform light distribution at substrate level (in the absence of a mask).
Many different factors can affect the uniformity of the light produced at the substrate level. For example, the uniformity is dependent on factors such as the presence of a diaphragm e.g. so called REMA (reticle masking) blades, films formed on optical elements e.g. anti-reflective coatings, water, organic films formed on the illuminator, back reflections and contamination. Therefore, it is desirable if the effect of these factors on the light distribution at substrate level can be minimised.
It is known to place devices in the path of the employed radiation beam to vary the light intensity incident on the reticle. For example, U.S. Pat. No. 5,486,896 uses a device which decreases the light intensity at the sides of the beam. This device has two diaphragms which intercept the edges of the beam. In one embodiment, small filter strips are placed on the inner edges of the diaphragm to create a smoother transition from light to dark at the edges of the beam.
This known device was designed to solve a problem which occurs in some scanning systems where the edges of the beam overlap when two adjacent sections of the wafer are scanned (so-called stitching problems). This results in overexposure at the edges of the scanning region.
The problems addressed by the present invention are far more complex and cannot be solved by just attenuating the light signal at the edges of the beam. The present invention is aimed at the requirement to modulate the spatial beam intensity along a given direction of the beam cross-section.
The present invention provides a device for optimizing the uniformity of an image in a lithographic projection system comprising: a radiation system for supplying a projection beam of radiation, a first object table provided with a mask holder for holding a mask, and connected to first positioning means, a second object table provided with a substrate holder for holding a substrate and connected to second positioning means, a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
In a first aspect, the present invention provides a lithographic projection apparatus having:
a radiation system comprising a source of radiation, for supplying a projection beam the cross-section of which, in the plane of a mask, is smaller than the mask pattern;
a mask table being movable in at least a first (scan) direction and being provided with a mask holder in which the mask can be held;
a projection system for imaging an irradiated part of the mask pattern onto the substrate with a magnification M;
a substrate table which is movable along the first direction and along a second direction perpendicular thereto, and is provided with a substrate holder in which the substrate can be held;
mask table drive device for moving the mask table during each imaging operation along at least the first direction, with respect to the projection beam and the projection system; and
substrate table drive device for moving the substrate table during each imaging operation along at least the first direction at a rate equal to M times the rate at which the mask table is moved.
The lithographic projection apparatus further comprises a correction device, the correction device comprising a filter unit placed in the path of the beam for varying the spatial intensity of the beam along at least one direction of its cross-section, so that the integrated radiation intensity at substrate level is substantially uniform across substantially the entire length of that cross-section.
In a second aspect, the present invention provides a correction device for a lithographic projection apparatus, wherein the lithographic projection apparatus comprises a projection system that projects a beam of radiation onto a mask. The correction device comprises a filter unit placed in the path of the beam for varying the spatial intensity of the beam along at least one direction of its cross-section so that the integrated radiation intensity at substrate level is substantially uniform across the entire length of that cross-section.
For the avoidance of doubt, as used herein, the term xe2x80x9cfilter unitxe2x80x9d refers to any device that blocks or partially blocks and/or reduces the intensity of the incident radiation. The term is not intended to be limited to device for blocking certain radiation energies or frequencies.
References to the spatial uniformity of the beam at substrate level of course refer to the uniformity in the absence of projected mask features.
The term substantially uniform preferably means a percentage variation in the uniformity of less than 1.5%. More preferably, the percentage variation is less than 0.5%.
In general, the beam cross-section will have the form of an elongated rectangle (e.g. with dimensions of the order of 10 mmxc3x9726 mm). A so-called xe2x80x9creference directionxe2x80x9d is now defined as being the direction of the long side of such a cross-section. In alternative arrangements, the beam cross-section may, for example, be square or circular; in such cases, the reference direction may be taken as being parallel to one of the sides of the square or one of the diameters of the circle, respectively.
Preferably, the filter unit comprise a first member which is located in the path of the beam. The integrated intensity of the radiation is modulated by moving the first member with respect to the beam along a movement axis. Movement of the first member refers to movement of the first member in its entirety or movement of a part of the first member with respect to the beam.
In a first preferred embodiment of a lithographic projection apparatus according to a first aspect of the present invention, and a correction device according to a second aspect of the present invention, the first member is a single filter. The filter is placed in the path of the beam and oriented such that there is a variation in its transmittance with respect to directions parallel and perpendicular to the movement axis. The filter is moved in its entirety with respect to the beam. In a preferential embodiment, the movement axis is perpendicular to the reference direction.
Thus, the filter can be provided with a plurality of specific transmission profiles arranged parallel to one another, each transmission profile being arranged perpendicular to the movement axis, such that a particular transmission profile can be chosen by moving the filter along the movement axis. Thus, the uniformity of the integrated intensity of the beam cross-section can be dynamically corrected by the movement of the first filter.
Preferably, the plurality of specific transmission profiles provides a continuum of constantly varying transmission profiles, i.e. the plurality of profiles defines a substantially continuously differentiable function with respect to spatial variables taken along the said direction of movement and a direction perpendicular thereto (Y and X axes). The particular function is preferably the reciprocal of the integrated radiation intensity sampled at (differential) intervals parallel to the reference direction.
The first filter is a continuous member, such that the whole cross-sectional area of the beam is incident on the filter member. In other words, the filter does not partially intersect the beam; if it did, there is a chance that light could be reflected from the edges of the filter which would further affect the uniformity of the integrated intensity of the beam at substrate level.
When the filter is moved to place a new transmission profile in the path of the beam, the filter is still positioned so that an edge of the filter does not intersect the beam. Hence, no spurious reflections due to the correction device itself are caused.
In a second embodiment, the first member has two filters. The second filter is movable along the said movement axis. The second filter is movable relative to said first filter.
It is even more preferable if the second filter overlies the first filter such that at least one transmission profile on each of the first and second filters overlap one another. The first and second filters have linear transmission profiles and the filters are oriented such that the linear transmission profiles on the first filter slope in opposite direction to the linear transmission profiles on the second filter. Put another way: when viewed in a direction perpendicular to the plane of the beam cross-section, the second filter is a mirror image of the first filter. In this way, the pupil (slit) illumination remains symmetrical, so as to overcome telecentricity problems.
Preferably either or both of the filter members are grey filters (absorption, continuous, reflection or dithered).
More preferably, the lithography apparatus further comprises a condenser lens group or REMA lens which is located between the mask and the source of the radiation, the first member being placed between the condenser lens group or REMA lens and the mask. Alternatively, the first member can be placed on the side of the mask remote from the source of radiation.
More preferably, the first member is located immediately above the level of the mask or at an optically conjugate position.
Preferably, for a filter member composed of discrete dots, such as a dithered filter comprised of for example 50-micron randomly distributed dots, the member is located at a defocused position to average out the dots at the mask or substrate level.
Even more preferably, the device further comprises a control unit for controlling the movement unit, responsible for effecting movement of the first member, such that the first member is positioned to optimise the uniformnity of the spatial intensity of the beam incident on the substrate.
Even more preferably, the control unit provides continuous control of the uniformity of the spatial intensity of the beam incident on the substrate by continually moving the filter in such a way that the best possible uniformity can be achieved at all times.
In a third embodiment, the first member comprises a plurality of independently movable fingers, the fingers being mutually adjacent and movable independently relative to each other, back and forth, to intersect at least part of the beam cross-section. To clarify, the fingers are arranged side-by-side along the reference direction, the fingers being movable perpendicular to this direction.
Preferably, the elongate fingers are oriented substantially perpendicular to the reference direction.
Preferably, the width of each finger in a direction parallel to the reference direction is chosen so as to constitute from 3% to 7% of the length of the slit in the reference direction. In that case, there will be of the order of about 15 to 25 fingers side-by-side along the reference direction. For example, in the case of a slit measuring 10 mm by 26 mm, one can chose each finger to be 1 mm long, so that there are 26 fingers side-by-side along the reference direction.
Means for moving the above plurality of fingers will be apparent to a person skilled in the art. For example, each finger could be directly or indirectly attached to a small motor.
A further problem addressed by the present invention is to provide an improved filter. As explained above, filters in which the transmission varies with position may be used to improve illumination uniformity and hence CD uniformity, and may also be used in many other applications. Often the transmission profiles can be rather complex. Previously, asymmetric filters (filters without rotational symmetry) have been xe2x80x9ccontinuousxe2x80x9d or analogue filters, in which the transmission profile changes gradually in a continuous way with position on the filter plate. The transmission is tuned either by varying the thickness of a dielectric coating (i.e. tuning the reflectance) or by varying the thickness of an absorbing coating (e.g. a metal layer). However, in practice it is very difficult to control layer thickness, especially when the thickness varies with position. As a result, filter transmission profiles often have the wrong profile and/or amplitude.
In order to alleviate this problem, the invention provides a filter (also called a digital filter) in which the transmission varies with position through varying the density of radiation-blocking dots.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of apparatuses according to the invention in the manufacture of ICs, it should be explicitly understood that such apparatuses have many other possible applications, especially where exposure conditions vary (e.g. Numerical Aperture, degree of coherence (i.e. sigma setting), degradation of the optics, etc). For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.